Image sensor and method of driving transfer transistor of image sensor

ABSTRACT

Provided is a 4-transistor CMOS image in which a driving condition or a pixel structure is changed so that a transfer transistor in a pixel operates in a pinch-off condition during reset and transfer operations in order to reduce dark current and fixed-pattern noise caused by a change in an operation condition of the transfer transistor and inter-pixel characteristic discrepancy. The image sensor includes a photosensitive pixel including a transfer transistor for transferring photon-induced charges created in a photodiode; and a voltage control unit for controlling a turn-on voltage applied to a gate of the transfer transistor to be lower than a floating diffusion node voltage plus the threshold voltage of the transfer transistor during a partial or entire section of a turn-on section of the transfer transistor such that the transfer transistor operates in a pseudo pinch-off mode.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 2005-119493, filed Dec. 8, 2005, and 2006-98889, filedOct. 11, 2006, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to an image sensor and a method of drivinga transfer transistor of the image sensor that transfers photon-inducedcharges, and more particularly, to an image sensor and a method ofdriving a transfer transistor of the image sensor that are capable ofmaintaining a depletion degree of charges in a photodiode when thephotodiode is reset.

2. Discussion of Related Art

Image sensors may be classified into a charged coupled device (CCD)sensor and a complementary metal oxide semiconductor (CMOS) imagesensor, which use electron-hole pairs separated by light having anenergy higher than a silicon band gap, in which an amount of irradiatedlight is generally estimated by accumulating either electrons or holes.

The CMOS image sensor includes image pixels each having a photodiode andtransistors, similar to a typical CMOS device. Image-signal processingand detecting circuits can be integrated in an external block of thepixel. This eliminates a need for an image-signal processing circuitincluded in a separate chip, allows a variety of image sensor structuresto be adopted, and provides flexibility so that subsequent imageprocessing is performed by hardware.

A 4-transistor pixel structure widely used to implement a CMOS imagesensor is shown in FIG. 1. The 4-transistor pixel structure is composedof four transistors. A photodiode PD that is a photo-sensing unit andfour NMOS transistors constitute one unit pixel. Among the four NMOStransistors, a transfer transistor Tx serves to transfer photo chargesgenerated by the photodiode PD to a diffusion node region 131, a resettransistor Rx serves to discharge charges from the diffusion node region131 or the photodiode PD so that a signal is detected, a drivetransistor Dx serves as a source follower transistor, and a switchingtransistor Sx is used for switching/addressing. The transfer transistorTx may be implemented by a gate, a gate oxide layer, and a p-typesubstrate, the photodiode PD may be generally implemented by an n- orno-doped region and a surface p-doped region, and the diffusion node 131may be implemented by an n+ doped region.

In FIG. 1, the photodiode PD receiving light and a capacitor 118connected parallel to the photodiode PD constitute a light-receivingunit, and the transfer transistor Tx serves to transfer electronsgenerated by photons to the diffusion node 131.

The transfer transistor Tx serves as a transmission channel thattransfers electrons generated from a surface of the photodiode PD to thediffusion node 131 or performs a reset function to completely removeelectrons from the photodiode PD in response to a voltage applied to itsgate. The diffusion node 131 includes a diffusion capacitor 114 and agate capacitor of the drive transistor Dx. The diffusion node 131 isreset by the reset transistor Rx. That is, the diffusion node 131 isreset before receiving the electrons from the photodiode PD region, or areset voltage is applied to the diffusion node 131 to reset thephotodiode PD region. A voltage is applied to a gate 141 of theswitching transistor Sx in order to select one row for a two-dimensionalimage. Each pixel is biased by a current source 150, which activates thedrive transistor Dx and the switching transistor Sx so that a voltage atthe diffusion node 131 is read out to an output node 142.

In a CMOS image sensor having the 4-transistor pixel shown in FIG. 1,photon-induced carriers accumulated in the photodiode after thephotodiode is reset are transferred to the floating diffusion node,causing a voltage drop across the diffusion node, and thus the voltagedrop is used to detect the amount of the photon-induced carriers. Inthis case, the transfer transistor must perform uniform reset andtransfer operations in order to accurately and uniformly detect theamount of the accumulated photon-induced carriers. A variety ofstructures of a conventional 4-transistor pixel including a fully resetpinned photodiode to allow a transfer transistor to perform uniformreset and transfer operations are disclosed. The pinned photodiode usesa state where all movable charges in the photodiode are completelydepleted and a voltage is not changed any more. Ideally, a photodiodevoltage is always pinned into a constant value irrespective of anexternal bias such as a voltage at the floating diffusion node.Therefore, the reset and transfer conditions for the transfer transistorbecomes always constant.

However, in the conventional CMOS image sensor having a 4-transistorpixel, a reduced operating voltage or a changed process condition mayalways change reset and transfer conditions depending on a relationshipbetween the gate voltage of the transfer transistor and the voltage atthe floating diffusion node.

Specifically, in a conventional driving method using a power supplyvoltage (VDD) as a transistor turn-on voltage, when the transfertransistor is reset, a voltage at the floating diffusion node is equalto the gate voltage VDD of the reset transistor minus a thresholdvoltage value threshold voltage (Vth) of the reset transistor RX(VDD−Vth). This value automatically allows a difference between the gatevoltage VDD of the transfer transistor and the voltage at the floatingdiffusion node to be equal to the threshold voltage Vth. Generally,since the reset and transfer transistors are formed in the same dopingcondition on a substrate, they have a similar threshold voltage Vth. Inthis case, a state in the condition corresponds to an edge between apinch-off state region, in which a transfer transistor's edge at thefloating diffusion node begins to be turned on according to thedefinition of the threshold voltage Vth, and a linear operation region.At a time when the transfer transistor's edge at the floating diffusionnode is turned on, a certain amount of electrons may promptly move fromthe floating diffusion node to the channel region of the transfertransistor. Accordingly, the voltage at the floating node issignificantly changed due to the capacitance. Furthermore, the amount ofelectrons from the floating diffusion node significantly changes in asmall difference in threshold voltage between the transfer transistorand the reset transistor. Such a nonuniform amount of electrons from thefloating diffusion node causes irregularity of the reset condition, thusdeteriorating the quality of an image.

Unstable reset and transfer operations of the transfer transistor maycause two typical problems of increased dark current and increased fixedpattern noise.

In the reset operation, since the reset transistor Rx is turned on thefloating diffusion node has a low impedance with respect to the ground,the voltage is substantially the same VDD−Vth as the power supplyvoltage VDD. In the transfer operation, since the reset transistor Rx isturned off and the floating diffusion node has a high impedance withrespect to the ground, electrons in the channel of the transfertransistor flow into the floating node (clock feedback), so that thevoltage at the floating node becomes lower than the voltage VDD−Vth.Additionally, the gate voltage of the transistor increases an ON voltageaccording to boosting condition. In this process, the floating nodevoltage differs between the reset and transfer operations. Thisdifferent voltage conditions have not caused any trouble because acompletely depleted (i.e., completely reset) pinned photodiode isemployed, i.e., the pixel is driven after the photodiode is completelydepleted. The use of the pinned photodiode can also suppress darkcurrent and other noises.

However, as a modern semiconductor process and device is scaled down andan operating voltage is reduced, the floating diffusion node voltagegets gradually lower. Accordingly, a pinning voltage of a pinnedphotodiode gets lower, thereby deteriorating a pixel characteristic suchas well capacity.

Further, a voltage barrier necessarily exists between the pinnedphotodiode and the channel of the transfer transistor to some extent. Tosuppress the effect of the barrier when the transfer transistor isturned on, a pinning voltage is made significantly different from thevoltage at the floating diffusion node. When the barrier is notsufficiently reduced, the pinned photodiode is not completely reset,which may cause more severe problems. That is, when an operating voltageindicated as the power supply voltage VDD is reduced, a differencebetween the pinning voltage and the floating diffusion node voltage isreduced. In addition, the well capacity may be lowered and resetting(e.g., depletion) may be insufficient.

To solve the problems, in a conventional technique, a voltage at afloating diffusion node forcibly rises from a typical voltage VDD−VTH tothe power supply voltage VDD using a boosting circuit. In anotherconventional technique, the floating diffusion node voltage rises to thepower supply voltage VDD sufficiently and quickly using a resettransistor Rx of a PMOS type, not a conventional NMOS type.

However, the voltage boosting circuit applies a voltage over a normaloperation condition, which may degrade the reliability of a gate oxide.When a PMOS transistor is used as a reset transistor Rx, it occupies awider area than an NMOS transistor. Accordingly, a fill factor isreduced to deteriorate a characteristic of the device, and two timesmore noise than in an NMOS transistor is generated, as known in the art.Further, this approach has a limitation of characteristic enhancement ina complete reset condition.

SUMMARY OF THE INVENTION

The present invention is directed to an image sensor and a method ofdriving a transfer transistor of the image sensor that are capable ofeffectively suppressing a noise such as dark current while reducingdependency on a state of a photodiode.

The present invention is also directed to an image sensor and a methodof driving a transfer transistor of the image sensor that are capable ofperforming reset and transfer operations even though a photodiode is notcompletely reset.

The present invention is also directed to an image sensor and a methodof driving a transfer transistor of the image sensor in which acharacteristic of a photodiode can be improved even when the photodiodeis designed as a completely reset type.

The present invention is also directed to an image sensor and a methodof driving a transfer transistor of the image sensor in which a transfertransistor operates in a pseudo pinch-off state.

The present invention is also directed to implementation of an imagesensor and a method of driving a transfer transistor of the image sensorthat are capable of effectively suppressing noise such as dark currentin low operating voltage and/or at low cost.

One aspect of the present invention provides a method of driving atransfer transistor of an image sensor including: applying a pseudopinch-off voltage to a gate of the transfer transistor in order to reseta photodiode; applying a turn-off voltage to the gate of the transfertransistor to block the photodiode during light accumulation; andapplying a transfer voltage to the gate of the transfer transistor inorder to transfer photon-induced charges accumulated in the photodiode.

One aspect of the present invention provides a method of driving atransfer transistor of an image sensor including: applying a resetvoltage to a gate of the transfer transistor in order to reset aphotodiode; applying a turn-off voltage to the gate of the transfertransistor to block the photodiode during light accumulation; andapplying a pseudo pinch-off voltage to the gate of the transfertransistor in order to transfer photon-induced charges accumulated inthe photodiode.

One aspect of the present invention provides an image sensor including:a photodiode; and a transfer transistor for transferring photon-inducedcharges created by the photodiode to a floating diffusion node, whereinthe floating diffusion node and a channel of the transfer transistor areseparated by a depletion region when the transfer transistor is turnedon, so that the transfer transistor operates in a pseudo pinch-off mode.

Here, the floating diffusion node and the channel of the transfertransistor may be separated by adjusting a process condition or layoutfor an image sensor manufacturing process.

Particularly, when the sensor comprises a reset transistor for resettingthe floating diffusion node, the threshold voltage of the transfertransistor or the reset transistor may be changed through a processcondition modification so that the transfer transistor operates in apseudo pinch-off mode, or the threshold voltage of the transfertransistor may be made higher than the threshold voltage of the resettransistor by ion implantation or oxide layer thickness adjustment.

The floating diffusion node and the channel of the transfer transistormay be separated by the depletion region by controlling a channelvoltage of the transfer transistor to be lower than a voltage at thefloating diffusion node during a partial or entire section of theturn-on section of the transfer transistor. That is, signal voltagesapplied to transfer transistor and/or the floating diffusion node may beadjusted to implement the pseudo pinch-off mode.

In this case, the image sensor may comprise a transfer transistor fortransferring photon-induced charges from the photodiode to the floatingdiffusion node, wherein the channel voltage of the transfer transistoris controlled lower than a voltage at the floating diffusion node duringa partial or entire section of the turn-on section of the transfertransistor such that the transfer transistor operates in a pseudopinch-off mode.

The channel voltage of the transfer transistor may be equal to theturn-on voltage applied to the gate minus the threshold voltage of thetransfer transistor. Accordingly, in order to control the channelvoltage of the transfer transistor to be lower than the voltage at thefloating diffusion node, the sensor may comprise a voltage control unitfor controlling a turn-on voltage applied to the gate of the transfertransistor to be lower than the floating diffusion node voltage plus thethreshold voltage of the transfer transistor.

Accordingly, the channel voltage of the transfer transistor that is thegate turn-on voltage minus the threshold voltage of the transfertransistor becomes lower than the voltage at the floating diffusionnode, thereby blocking electrons from flowing from the floatingdiffusion node to the transfer transistor channel.

Methods, at the voltage control unit, of controlling the turn-on voltageapplied to the gate of the transfer transistor to be lower than thefloating diffusion node voltage plus the threshold voltage of thetransfer transistor may include a method of applying a switching signalhaving a lower pseudo pinch-off voltage than a normal gate turn-onvoltage to the gate of the transfer transistor so that the gate turn-onvoltage of the transfer transistor is lowered while a bias voltage ofthe reset transistor remains normal; and a method of applying a slightlyhigher reset voltage than the power supply voltage to the gate and/orsource of the reset transistor and applying a normal turn-on voltage tothe transfer transistor in order to increase a voltage applied to thefloating diffusion node.

Unlike a conventional technique adapted to fully deplete a photodiode,in the present invention, the amount of charge remaining in the channelregion of the transfer transistor is not affected by other charges whenthe transfer transistor is turned on. Accordingly, when the photodiodeis transfer state, the amount of charge remaining in the photodiode getsconstant, and an influence on the amount of charge in the photodiode andthe amount of charge transferred by the transfer transistor isminimized, thus reducing noise and dark current. In the presentinvention, when turned on, the transfer transistor operates at apredetermined pseudo pinch-off voltage so that the amount of chargeremaining in the source of the transfer transistor is unchanged.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent to those of ordinary skill in the art bydescribing in detail preferred embodiments thereof with reference to theattached drawings in which:

FIG. 1 is a circuit diagram illustrating a structure of a typical4-transistor CMOS image sensor;

FIG. 2 is a cross-sectional view illustrating a photodiode and transfertransistor region of a CMOS image sensor according to an exemplaryembodiment of the present invention;

FIG. 3 is a cross-sectional view illustrating a photodiode and transfertransistor region of a CMOS image sensor according to another exemplaryembodiment of the present invention;

FIG. 4 is a timing diagram illustrating a method of driving a transfertransistor of a conventional image sensor;

FIG. 5 is a timing diagram illustrating a method of driving a transfertransistor of an image sensor according to an exemplary embodiment ofthe present invention;

FIG. 6 is a timing diagram illustrating a method of driving a transfertransistor of an image sensor according to another exemplary embodimentof the present invention;

FIG. 7 is a timing diagram illustrating a method of driving a transfertransistor of an image sensor according to still another exemplaryembodiment of the present invention;

FIG. 8 is a timing diagram illustrating a method of driving a transfertransistor of an image sensor according to yet another exemplaryembodiment of the present invention;

FIG. 9 is a timing diagram illustrating a method of driving a transfertransistor of an image sensor according to yet another exemplaryembodiment of the present invention; and

FIG. 10 is a block diagram illustrating a structure of a CMOS imagesensor according to an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention will bedescribed in detail. However, the present invention is not limited tothe embodiments disclosed below, but can be implemented in variousforms. The following embodiments are described in order for thisdisclosure to be complete and enabling of practice of the invention bythose of ordinary skill in the art.

Although in the following embodiments of the present invention, thephotosensitive pixel will be described as being applied to a4-transistor CMOS image sensor, it may be applied to other imagesensors, e.g., a low-voltage output sensing circuit of a CCD, comprisinga photodiode and a transistor for transferring photon-induced chargesgenerated by the photodiode without departing from the scope of thepresent invention.

First Exemplary Embodiment

An image sensor of this embodiment includes: a photodiode; aphotosensitive pixel including a transfer transistor for transferringphoton-induced charges created in the photodiode to a floating diffusionnode; and a voltage control unit for applying a switching signal havinga pseudo pinch-off voltage lower than the power supply voltage or arising time that is two times or greater than the minimum rising time,to the gate of the transfer transistor during a partial or entiresection of a turn-on section of the transfer transistor. Preferably, thepseudo pinch-off voltage ranges from a value several hundreds of mVlower than the power supply voltage to a half the power supply voltage.

In the case where this embodiment is implemented by a CMOS image sensor,it may be applied to the 4-transistor pixel structure shown in FIG. 1.Once a pseudo pinch-off voltage value to be generated by a voltagecontrol unit is determined, the control unit may be implemented using avariety of conventional ways. The voltage control unit will be brieflydescribed later.

FIG. 2 shows a photodiode and transfer transistor of a CMOS image sensorhaving a typical structure to which a method of driving a transfertransistor according to the exemplary embodiment may be applied. In thisembodiment, a switching signal applied to a gate electrode of thetransfer transistor is adjusted in order to obtain optimal performanceof the sensor.

FIG. 4 shows a switching signal applied to a gate of a conventionaltransfer transistor. As shown in FIG. 4, a switching signal RxP of areset transistor and a switching signal TxP_P of the transfer transistorhave the same turn-off level (Von and Voff). Here, Von indicates thepower supply voltage VDD of FIG. 1 and Voff indicates a ground voltage.

FIG. 5 shows a switching signal applied to a gate of a transfertransistor according to the exemplary embodiment. The switching signalincludes a photodiode reset section 342, a diffusion node reset section344, a photon-induced electron accumulation section 348, and a section346 in which electrons accumulated in a photodiode is transferred to adiffusion node. The signal further includes a read section 349 in whichseveral pixels are sequentially read. The read section 349 is generallyshorter than the photon-induced electron accumulation section 348.

A method of driving a transfer transistor of the image sensor accordingto this embodiment using the shown waveform includes the steps of:applying a pseudo pinch-off voltage to the gate of the transfertransistor in order to reset a photodiode; applying a turn-off voltageto the gate of the transfer transistor to block the photodiode duringaccumulation; and applying a transfer voltage to the gate of thetransfer transistor in order to move photon-induced charges accumulatedin the photodiode. In a case of a 4-transistor pixel, the method mayfurther include the step of applying a turn-off voltage to the gate ofthe transfer transistor while the transferred photon-induced charges arebeing read after applying the transfer voltage.

The pseudo pinch-off voltage is applied in the photodiode reset section342, the turn-off voltage is applied in the photodiode accumulationsection 348, the transfer voltage is applied in the photon-inducedelectron transfer section 346, and the turn-off voltage is applied inthe photodiode read section 349 in read operation.

Applying the pseudo pinch-off voltage corresponds to a technical spiritof the present invention in which in a pseudo pinch-off state, chargesin the channel of the transfer transistor are not affected by charges inother portions, e.g., charges in the floating node. When a high voltageis applied to the transfer transistor, the channel of the transfertransistor is connected to the floating node so that charges in thefloating node flow into the channel of the transfer transistor. Thisaffects resetting the transfer transistor and moving the photon-inducedcharges and thus deteriorates several characteristics of a pixel.

As the pseudo pinch-off voltage is applied using the turn-on voltage ofthe transfer transistor, the above problems can be solved and theperformance of the image sensor can be prevented from being deteriorateddue to deviations in a manufacture process.

Meanwhile, when the reset transistor and the transfer transistor havethe same threshold voltage, the pseudo pinch-off voltage is slightly(preferably, 0.1V to VDD/2) lower than a gate voltage of the resettransistor. This pseudo pinch-off voltage enables reset operation andphoton-induced charge transfer operation. Accordingly, the amount ofcharge in the channel of the transfer transistor is less affected by theamount of other charges, e.g., the amount of charge in the floatingnode, as described above. In a conventional implementing method, a powersupply voltage VDD is applied to both gates of the reset transistor andthe transfer transistor, which implies that since the transfertransistor and the reset transistor have the same threshold voltage, theamount of charge in the channel of the transfer transistor issufficiently affected by the amount of charge in the floating node.

When a conventional method of applying a driving signal is used, thesame effect can be obtained by changing a threshold voltage of thetransfer transistor and the reset transistor. Particularly, the gatevoltage of the transfer transistor gets smaller than that of the resettransistor by making the threshold voltage of the transfer transistorhigher than that of the reset transistor by modifying a manufacturingprocess or bias, such that the turn-on voltage applied to the gate ofthe transfer transistor is forcibly lower than the voltage of thefloating diffusion node plus the threshold voltage of the transfertransistor.

Further, since the amount of the non-reset charge is always constant dueto a nature of the pseudo pinch-off state and is less affected by otherenvironmental factors, a constant value corresponding to the amount ofthe non-reset charge may be cancelled to obtain a more accuratephotosensitive value. However, a too low gate voltage of the transfertransistor obstructs sufficiently reducing a barrier between thephotodiode and the channel of the transfer transistor, thus degradingtransferring or resetting capability.

A term pseudo pinch-off state refers to a state physically similar witha pinch-off mode of operation of the MOS transistor. Preferably, apseudo pinch-off gate voltage for obtaining the pseudo pinch-off stateranges from a hundreds of mV lower value than the power supply voltagecorresponding to VDD of FIG. 1 to a half the power supply voltage.

In FIG. 5, in the electron transferring section 346, the pseudopinch-off voltage may be applied to the gate of the transfer transistor,such that the pseudo pinch-off state is stabilized, and the amount ofcharge that do not move from the photodiode to the floating diffusionnode remains unchanged. Thus, it is possible to increase the accuracy ofthe photo electrons transferring operation. In addition, the amount ofremaining charge upon resetting and the amount of remaining charge uponreading get constant, and a separate CDS circuit may not be required.Thus, it is possible to more simply manufacture a CMOS image sensor. Inthis case, the pseudo pinch-off voltage in the photo electronstransferring section 346 gets the same as or different from the pseudopinch-off voltage in the reset section 342. In the reset section 342,the reset voltage of the transfer transistor can range from VDD to VDD/2

Alternatively, in the transfer operation, the power supply voltage VDDor another voltage may be applied in the reset section 346, as in aconventional technique, in order to obtain sufficient charges from thephotodiode.

Meanwhile, in the case where the reset operation can be sufficientlyaccomplished using the pinned photodiode, a normal turn-on voltage maybe applied in the reset section 342 and the pseudo pinch-off voltage maybe applied only in the photo electrons transferring section 346.

Meanwhile, a rising time (i.e., leading time) of the switching signalwaveform affects the pseudo pinch-off state. The greater the risingtime, the more a reduced amount of the voltage applied to the transfertransistor gate can be reduced. That is, the signal having a small slopemakes the pseudo pinch-off state more stable.

FIG. 6 shows another embodiment of a switching signal gate to apply asmoother pseudo pinch-off voltage in order to obtain a more stabilizedpseudo pinch-off state. Upon shifting from section 349 to section 342,the switching voltage TxP_1 of the transfer transistor rises from aturn-off voltage Voff to a pseudo pinch-off voltage Vpo. In this case, aseparate delay circuit increases the rising time of the switching signalto be two times the rising time of an output signal of a normalswitching control unit (hereinafter, referred to as a minimum risingtime) unless a signal output means cannot work.

FIG. 7 shows another embodiment of a switching signal to apply astep-shaped pseudo pinch-off voltage to a gate in order to obtain a morestabilized pseudo pinch-off state. Upon shifting from section 349 tosection 342, the switching voltage TxP_1 of the transfer transistorrises from a turn-off voltage Voff to a pseudo pinch-off voltage Vpo. InFIG. 7, the switching signal rises to a first pseudo pinch-off voltageVpo1 and then to a second pseudo pinch-off voltage Vpo2 after apredetermined time elapses. The switching signal may be applied insteps, such as in three or more steps. Alternatively, the switchingsignal may rise in steps to the power supply voltage VDD as a finalvalue. Even when the gate voltage sequentially rises in steps to thesame level as the power supply voltage VDD, the transfer transistorremains in a pseudo pinch-off state to some extent.

This implementation is intended to reduce a risk of no pseudo pinch-offstate when the gate voltage of the transfer transistor rapidly rises.Generally, the switching signal of the transfer transistor may beadjusted in a multi-step by providing an additional function to adigitally-driven decoder for a row of a pixel. Driving the transfertransistor in the multi-level is helpful to make the transfer transistorin the pseudo pinch-off state. That is, in the methods of FIGS. 6 and 7,a rising width per hour of the switching signal applied to the gate ofthe transfer transistor is reduced to suppress the rise of the channelvoltage compared to the rise of the floating diffusion (FD) node voltageas much as possible, so that the transfer transistor stays in a pseudopinch-off region.

Although the final turn-on voltage has been shown in FIGS. 6 and 7 asbeing lower than a conventional turn-on voltage by allowing the turn-onto rise to a rising time that is two times or greater the minimum risingtime smoothly or in steps, the final turn-on voltage may be the same asthe conventional turn-on voltage (power supply voltage) in order toobtain the pseudo pinch-off state of the transfer transistor as intendedby the present invention. This is because when a rising speed of thevoltage applied to the gate of the transfer transistor is low, thechannel voltage follows the gate voltage with a constant delay time inwhich the transfer transistor becomes in the pseudo pinch-off state.

Although not shown in detail, the waveform of FIG. 7 may include all thefeatures of FIGS. 6 and 7. That is, two or more step-shaped pseudopinch-off voltages are sequentially applied and the rising time thereofmay expand by using a separate delay means.

FIG. 10 shows a structure of the image sensor including a photosensitivepixel and a relevant control block according to this embodiment. Avoltage control unit may be implemented by a pulse generating block2000.

The pulse generating block 2000 receives RX and TX signals applied to aconventional image sensor to generate Vrx and Vtx signals as shown inFIGS. 5 to 9, so that the turn-on voltage applied to the gate of thetransfer transistor gets lower than that of the floating diffusion node.

Second Exemplary Embodiment

An image sensor of this embodiment includes a transfer transistor fortransferring photon-induced charges created in the photodiode to afloating diffusion node, and a reset transistor for resetting thefloating diffusion node, in which, a voltage applied to a drain and/orgate of the reset transistor is changed to control the channel voltageof the transfer transistor to be lower than the floating diffusion nodevoltage so that the transfer transistor operates in a pseudo pinch-offmode.

Here, the turn-on voltage of the transfer transistor may be a powersupply voltage. Preferably, the voltage applied to the drain and/or gateof the reset transistor may range from a value several hundreds of mVhigher than the power supply voltage to a value 1.5 times higher thanthe power supply voltage.

When this embodiment is implemented by a CMOS image sensor, it may beapplied to the 4-transistor pixel structure shown in FIG. 1. It can beseen that even in the image sensor of this embodiment, a voltage at thefloating diffusion node is higher than that of a channel of the transfertransistor when the transfer transistor is turned on. The resultingoperation and effects have been sufficiently described in the firstembodiment and thus a description thereof will be omitted herein.

In this embodiment, when the gate turn-on voltage, which is the powersupply voltage as in a convention technique, is applied to the gate ofthe transfer transistor and the transfer transistor is turned on, thevoltage of the floating diffusion node increases for a pseudo pinch-offstate. Methods of resetting the floating diffusion node into a higherreset voltage than the power supply voltage as described above include amethod of increasing the gate voltage of reset transistor to be higherthan the power supply voltage when the transfer transistor is reset(i.e., turned on), and a method of increasing gate and drain voltages ofthe rest transistor to higher than the power supply voltage when thetransfer transistor is reset.

When a power supply voltage is applied to a gate and drain of an NMOStransistor to be turned on, a source voltage is the gate voltage minusthe threshold voltage. Accordingly, when the gate voltage increases, thesource voltage correspondingly increases to the drain voltage (powersupply voltage) as a limit. In the case where the source voltage shouldget higher than the limit, both gate and drain voltages of the resettransistor must increase. When the transfer transistor and the resettransistor have the same threshold voltage, only the turn-on gatevoltage of the reset transistor may increase to a hundreds of mV levelhigher than the power supply voltage.

FIG. 10 shows a structure of the image sensor including thephotosensitive pixel and the relevant control block according to thisembodiment. A voltage control unit is a positive (+) bias block 3000 inthe case of the former method and is a pulse generating block 2000 inthe case of the latter method.

In the case of the former method, the positive (+) bias block 3000 maybe implemented for example by a capacitor-based boosting circuit andgenerates a higher voltage than the power supply voltage and applies itto the drain of the reset transistor.

In the case of the latter method, the pulse generating block 2000 uses,for example, a boosting circuit to adjust the reset turn-on voltage Vrxapplied to the gate of the reset transistor Rx to be higher than thepower supply voltage. That is, the pulse generating block 2000 performssteps of applying a turn-on voltage higher than the power supply voltageto the gate of the reset transistor in order to reset the photodiode;and applying a turn-off voltage to the gate of the reset transistor inthe photo electrons accumulating section, the transferring section, andthe reading section.

Third Exemplary Embodiment

FIG. 3 shows a structure of a photodiode and a transfer transistor of aCMOS image sensor having an enhanced structure to which a method ofdriving a transfer transistor according to this exemplary embodiment maybe applied. In this embodiment, a switching signal applied to the gateof the transfer transistor includes a turn-on voltage that is enhancedas in the first embodiment, and a turn-off voltage that is a lowernegative offset voltage than a ground voltage in order to optimize theperformance of the image sensor.

A method of applying a pseudo pinch-off voltage and the resulting effectare the same as in the first embodiment, and thus the enhanced structureof the transfer transistor, a method of applying a negative offsetvoltage, and the resulting effect will be described herein.

Only the photodiode PD, the transfer transistor Tx, and the floatingdiffusion region 131 of the 4-transistor pixel are shown in FIG. 3. Thetransfer transistor Tx includes a gate 310, a gate oxide layer 320, anda p-type substrate 360, the photodiode region PD includes a photodiode(n) doped region 350 and a surface p doped region 330, and the diffusionnode 340 is an n+ type node. In this case, a p doped region 332contiguous to the transfer transistor is formed adjacent to the surfacep doped region 330. In this embodiment, holes are accumulated in the pdoped region 332 so that the performance of the image sensor isimproved.

In the accumulation section (348 of FIG. 8), a constant negative offsetvoltage Vos is applied to the gate of the transfer transistor 310, sothat holes are accumulated in the p doped region 332 contiguous to thetransfer transistor via the gate oxide layer 320. In this case, a trapis inactivated in the p doped region 332 contiguous to the transfertransistor and electrons-hole pairs are reduced. Thus, dark current isreduced. In addition, the gate 310 voltage applied while the transfertransistor is turned off increases the voltage barrier beneath the gateoxide layer 320 and, in turn, well capacity of electrons that can beaccumulated in the photodiode.

There may be several methods of manufacturing the transfer transistorhaving the shown structure. Among them, in a manufacturing methodcapable of minimizing a modification of a conventional image sensormanufacturing process, a p-type layer 330 of a typical photodiode isformed by forming a gate oxide and implanting a p-type dopant, such asboron, into the gate oxide. In this case, a subsequent annealing processcauses a boundary of the implanted dopant to be diffused below the gateoxide. Researchers have tried to minimize the diffusion of the dopant.However, in this embodiment, the p-type region 332 overlapping the gateelectrode 310 is formed by maximizing the diffusion of the dopant. Thatis, the region 332 overlapping the gate electrode 310 among the twop-type layers 330 and 332 of the photodiode is formed by horizontaldiffusion of the dopant.

In another method of manufacturing a transfer transistor of thisembodiment, the p-type region 332 overlapping the gate electrode 310 maybe formed to be integral with the p-type region 330 of the photodiode ormay be formed independently, through a separate lithographic process anda subsequent stacking process before the gate electrode 320 is formed.

In the latter case, the p-type doping region 332 is doped in a differentpattern from the surface p doped region of the photodiode. In general,the surface p doped region of the photodiode is formed by somewhatcomplex doping, including double or more doping, for photosensitiveefficiency and/or reset efficiency. Since the p-type doping region 332is for increasing hole accumulation and charge transferal efficiencies,it should be doped in an advantageous manner to the efficiencies. Doubledoping may be unnecessary and a small thickness as in the surface p-typeregion 230 may be not required. In the latter case, a trap removaleffect by the p doped region 332 beneath the gate oxide 320 can bemaximized.

An example of a switching signal waveform for the enhancement of thetransfer transistor and the advantages of the present invention is shownin FIG. 8. The waveform of the switching signal applies a negative (−)offset voltage Vos, not a ground voltage Voff, to the gate of thetransfer transistor during the accumulation section 348 to accumulatephoton-induced electrons. Even in the read section 349, the offsetvoltage Vos is applied, but another voltage such as the ground voltageVoff may be applied. The negative offset voltage is determined to be themost excellent trap inactivation point between about −0.1V and −1.0V toobtain optimal performance.

The offset voltage of the present invention must be a negative voltagehaving a smaller absolute value than the power supply voltage andrequires a minor current, which makes it possible to easily configure acircuit for generating the offset voltage.

FIG. 9 shows a switching signal waveform including the enhancements ofFIG. 6 and FIG. 8. A negative offset voltage Vos is used as the turn-offvoltage of the transfer transistor, and a pseudo pinch-off voltage isused as the turn-on voltage. A rising time from the negative offsetvoltage Vos to the pseudo pinch-off voltage Vpo is delayed by a separatedelay means. An enhancement according to FIG. 9 can be inferred from thedescription of FIGS. 6 and 8.

Although in the third embodiment, the configuration according to thefirst embodiment and the configuration of applying the negative offsetvoltage have been combined, the configuration according to the secondembodiment, which is simpler than the first embodiment configuration,and the configuration of applying the negative offset voltage may becombined.

For example, other switching signal waveforms including two or morefeatures of the respective switching waveforms, including the switchingsignal waveforms shown in FIGS. 6 to 9, can be implemented, and adetailed description thereof can be inferred from the aforementioneddescription.

For example, although the present invention is implemented in acondition that the photodiode is partially depleted, it may beimplemented in a condition that the photodiode is pinned. That is, resetand transfer operations can be performed independent of a voltage at afloating diffusion node by operating the transfer transistor in a pseudopinch-off state even though the diode is pinned by a pinning voltage,thereby obtaining excellent characteristics.

According to the image sensor of the present invention, it is possibleto effectively suppress dark current and other noises even in lowoperating voltage.

It is also possible to operate the transfer transistor in a pseudopinch-off state without having to increase a voltage at the FD node.

It is also possible to easily improve performance of an image sensor bychanging switching signals applied to a photodiode and a transfertransistor.

It is also possible to suppress dark current and other noises withoutneed of a separate pinned photodiode having a fully depleted (or fullyreset) structure.

While the invention has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in forms and details may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims.

1. An image sensor comprising: a photodiode; and a transfer transistorfor transferring photon-induced charges created by the photodiode to afloating diffusion node, wherein the floating diffusion node and achannel of the transfer transistor are separated by a depletion regionwhen the transfer transistor is turned on, so that the transfertransistor operates in a pseudo pinch-off mode.
 2. The image sensor ofclaim 1, wherein the floating diffusion node and the channel of thetransfer transistor are separated by the depletion region by controllinga channel voltage of the transfer transistor to be lower than a voltageat the floating diffusion node during a partial or entire section of theturn-on section of the transfer transistor.
 3. The image sensor of claim2, further comprising a voltage control unit for controlling a turn-onvoltage applied to a gate of the transfer transistor to be lower thanthe floating diffusion node voltage plus the threshold voltage of thetransfer transistor.
 4. The image sensor of claim 3, wherein the voltagecontrol unit applies a switching signal having a pseudo pinch-offvoltage lower than the power supply voltage to the gate of the transfertransistor during a partial or entire section of the turn-on section ofthe transfer transistor
 5. The image sensor of claim 4, wherein thepseudo pinch-off voltage ranges from a value several hundreds of mVlower than the power supply voltage to a half the power supply voltage.6. The image sensor of claim 1, further comprising a voltage controlunit for controlling the gate voltage of the transfer transistor to havea rising time two times or greater than a minimum rising time when thegate voltage rises from a turn-off voltage to a turn-on voltage.
 7. Theimage sensor of claim 1, further comprising a reset transistor forresetting the floating diffusion node, wherein a turn-on gate voltagehigher than the power supply voltage is applied to a gate of the resettransistor, and the power supply voltage is applied as a turn-on gatevoltage of the transfer transistor.
 8. The image sensor of claim 7,wherein the turn-on gate voltage of the reset transistor ranges from avalue several hundreds of mV higher than the power supply voltage to avalue 1.5 times higher than the power supply voltage.
 9. The imagesensor of claim 7, wherein a drain voltage higher than the power supplyvoltage is applied to a drain of the reset transistor.
 10. The imagesensor of claim 9, wherein the drain voltage of the reset transistorranges from a value several hundreds of mV higher than the power supplyvoltage to a value 1.5 times higher than the power supply voltage. 11.The image sensor of any one of claims 1 to 10, wherein the transfertransistor has a structure capable of causing holes to be accumulated ina partial or entire region beneath a gate oxide layer.
 12. The imagesensor of claim 11, wherein the switching signal has a negative (−)offset voltage to the gate during a partial or entire section of theturn-off section of the transfer transistor.
 13. The image sensor ofclaim 12, wherein the negative offset voltage ranges from −0.1V to−1.0V.
 14. The image sensor of claim 11, wherein the transfer transistorcomprises: a p-type doping portion formed between a surface p-typeregion of the photodiode and a charge transmission channel from thephotodiode to the diffusion node, and having a different doping patternfrom the surface p-type region of the photodiode; a gate oxide layerdisposed on the p-type doping portion and the charge transmissionchannel; and a gate electrode disposed on the main gate oxide layer. 15.The image sensor of claim 1, wherein the floating diffusion node and thechannel of the transfer transistor are separated by adjusting a processcondition or layout during a manufacturing process of the image sensor.16. The image sensor of claim 15, further comprising a reset transistorfor resetting the floating diffusion node, wherein the threshold voltageof the transfer transistor or the reset transistor is changed through aprocess condition modification so that the transfer transistor operatesin a pseudo pinch-off mode.
 17. The image sensor of claim 15, furthercomprising a reset transistor for resetting the floating diffusion node,wherein the threshold voltage of the transfer transistor is made higherthan the threshold voltage of the reset transistor by ion implantationor oxide layer thickness adjustment.
 18. A method of driving a transfertransistor of an image sensor, the method comprising the steps of:applying a pseudo pinch-off voltage to a gate of the transfer transistorin order to reset a photodiode; applying a turn-off voltage to the gateof the transfer transistor to block the photodiode during lightaccumulation; and applying a transfer voltage to the gate of thetransfer transistor in order to transfer photon-induced chargesaccumulated in the photodiode.
 19. The method of claim 18, furthercomprising the step of applying a turn-off voltage to the gate of thetransfer transistor while the transferred photon-induced charges areread out.
 20. The method of claim 18, wherein the step of applying thepseudo pinch-off voltage comprises the step of gradually increasing thegate voltage of the transfer transistor from the turn-off voltage to thepseudo pinch-off voltage.
 21. The method of claim 18, wherein the stepof applying the pseudo pinch-off voltage comprises the step ofsequentially applying at least two pseudo pinch-off voltages from alower level to a higher level to the gate of the transfer transistor.22. The method of claim 18, wherein the transfer voltage is the same asthe pseudo pinch-off voltage.
 23. The method of claim 22, wherein thestep of applying the transfer voltage comprises the step of graduallyincreasing the gate voltage of the transfer transistor from the turn-offvoltage to the pseudo pinch-off voltage.
 24. The method of claim 22,wherein the step of applying the transfer voltage comprises the step ofsequentially applying at least two pseudo pinch-off voltages from alower level to a higher level to the gate of the transfer transistor.25. The method of any one of claims 18 to 24, wherein the pseudopinch-off voltage ranges from a value several hundreds of mV higher thanthe power supply voltage to a value 1.5 times higher than the powersupply voltage.
 26. A method of driving a transfer transistor of animage sensor, the method comprising the steps of: applying a resetvoltage to a gate of the transfer transistor in order to reset aphotodiode; applying a turn-off voltage to the gate of the transfertransistor to block the photodiode during light accumulation; andapplying a pseudo pinch-off voltage to the gate of the transfertransistor in order to transfer photon-induced charges accumulated inthe photodiode.
 27. The method of claim 26, further comprising the stepof applying the turn-off voltage to the gate of the transfer transistorwhile the transferred photon-induced charges are read out.
 28. Themethod of claim 26, wherein the step of applying the pseudo pinch-offvoltage comprises the step of gradually increasing the gate voltage ofthe transfer transistor from the turn-off voltage to the pseudopinch-off voltage.
 29. The method of claim 26, wherein the step ofapplying the pseudo pinch-off voltage comprises the step of sequentiallyapplying at least two pseudo pinch-off voltages from a lower level to ahigher level to the gate of the transfer transistor.
 30. The method ofany one of claims 26 to 29, wherein the pseudo pinch-off voltage rangesfrom a value that is several hundreds of mV lower than a power supplyvoltage to a half of the power supply voltage.
 31. The method of any oneof claims 26 to 29, wherein the reset voltage ranges from a value thatis from a power supply voltage to a half of the power supply voltage.